The invention designates a measuring probe of a measuring device and an associated measurement procedure for the measurement of the concentration of agents in gases and/or liquids. To do this, the measuring device uses the change in electrical characteristics. To conduct the measurement, the substances to be examined are placed in contact with the surface of the measuring device. The substance changes the conductivity, depending on its concentration, of the surface of the measuring device.
It is known, that the concentration of various agents can be determined with the aid of resistance measurements. For example, the international patent classification (IPC), GO1N 27/00, describes the analysis of materials through the application of electrical methods. Under GO1N 27/12 special resistance studies are considered, which show the change in the resistance of a solid in relation to the absorption of a liquid. Measuring probes with electrodes are used to do this, which are described in GO1N 27/07.
It is also known that such measuring probes of measuring devices consist of two suitably formed electrodes, which are attached to a support, and that the conductivity of a suitable substance is determined between these electrodes as an indirect measurement variable for the determination of the concentration of the substances being examined. Depending on the agents being examined, various often specially optimized substances are used, on which the agents being examined adsorb. In addition, the substance itself also has a certain electrical conductivity, which is changed by the adsorption and physical combination of agents serving as an adsorbate. Organic and inorganic semiconductor materials are used as suitable materials for this purpose, since the relative change of the conductivity due to the adsorption of these agents is sufficiently large. For various agents, both small band and broadband substances are known, which function selectively. The change of the electrical conductivity serves to determine the concentration since a monotone relation of this with the concentration exists in the applied measurement area. It is possible to use an oscillating field for the measurement of the change in the electrical conductivity and to apply its additional parameters such as specific complex loss angle as an additional aid for the evaluation.
By using a suitable design of the electrodes, the usable conductivity range can be represented by a suitable conductance of the measuring probe. By means of a suitable arrangement of the surface, such as pores, between the electrodes, the proportional influence of the adsorption on the part of the substance used for the measurement can be changed. The adsorption time is determined, in particular, by the type of substance layer and the substance temperature.
A number of such measuring probes for the determination of the concentration of various agents in gases are manufactured with organic semiconductor material, preferably polymers, on ceramic supporting materials. Due to the high humidity and the high surface tension of water, the substances of such measuring probes are coated with a thin water film in a normal climate. Due to its own conductivity, this results in a total conductivity of the measuring device, which is about one magnitude above that of the substances used. In order to prevent an incorrect measurement due to the absorption of water, such measuring probes are equipped with a heating element or a separate heating unit, which heats the substance so that the moisture film evaporates completely. These measuring probes necessarily function with higher temperatures compared to the surroundings and primarily above 150xc2x0 C. The useful measurement range, with respect to the concentration of the agents, normally runs from a minimum of 1 part per million (ppm) to a saturation concentration of the agent being determined. Within this measurement range and with increasing concentration, there is a monotone increasing conductance of the measuring probe, which, based on previous calibration, can be converted into the concentration of the agent being examined.
The disadvantage of such sensors is the relatively low sensitivity to very low concentrations for certain agents in U gases and the requirement for heating. Due to this, such measuring probes are more complex and more expensive to manufacture and to operate. In addition, the use of these in the ambient temperature range, for example for a normal climate, is limited.
The specification sheet, EP 0 328 108 A3, describes an electrochemical sensor for the measurement of the concentration of a chemical substance in a solution, where two field-effect transistors (FET) and a reference electrode are arranged on a substrate. A hydrogel as an xe2x80x9celectrodexe2x80x9d is arranged above the area of the channel of one of the FETs and the reference electrode and enzymes are used for substance detection, which activate the FET through the change of the conductivity in the electrode. The detection of the type and concentration of the substance in the solution is done by means of an evaluation of the signal from the FET.
This sensor can only be used for the determination of relatively high concentrations in the range of a few parts per million for substances in solutions but agents in gases cannot not be determined sufficiently with this method. The concentration of only a few selected substances can be determined in this manner. In addition, the hydrogel for the sensor can be easily and irreversibly contaminated with substances disturbing the measurement and this makes the sensor unusable. Due to the microstructures, expensive technologies from the microelectronics area are needed to manufacture such sensors.
The announcement of the PCT application with the number, WO 89/08713, reveals a method and a device for determining the concentration of certain body fluids. A fluid sample is placed in a sample cell with two electrodes and mixed with an oxidizing agent and a buffer as a redox-system and then the conductivity is read from an ammeter and an evaluation unit and display unit show the concentration of the substance in the body fluid. The conductivity of the sample fluid is also used to turn on the measuring device. The disadvantage of this method is the limitation to liquids, the relative insensitivity with a lower detection range in a concentration of ppm and the arrangement of a reference electrode in the sample cell. This is further development of the measurement arrangement for the determination of the conductivity of liquids.
The purpose of the invention is to develop a measuring probe and an associated measuring process, which results in a sensitive measuring probe, without the above disadvantages, for the detection of agents and their concentration in gases and/or liquids and this is to be done under the most varied of real measuring conditions without additional expenditure and without a heating element.
This purpose is fulfilled by the characteristics listed in patent claim 1 and in patent claim 7. Priority for further developments result from the subclaims.
The essence of the invention is that a measuring probe in the form of a dipole is used to determine the electrical resistance of a sensor-active layer, where a covering film from a liquid such as water is purposely included in the active zone of the measuring probe. The covering film forms over the substance of the sensor-active layer. Based on the covering film, a combination of various partial conductances is available. In particular these are the conductances for the substance, the covering film and for the active surface, which forms between both of these. Molecules of the covering film can also serve as an adsorbate for the agent to be determined in the gas or liquid, in addition to the substance. The adsorption characteristics for certain agents can be optimized by the systematic selection of the liquid for the covering film.
The measurement procedure, according to the invention, is specially designed for the measuring probe according to the invention. The probe operates basically under saturated conditions with respect to the adsorption of the liquid in the diffusion layer. In its basic condition (0% agent+liquid in saturation) there is a comparatively high conductance. The presence of certain agents, depending on the type, results in a hindrance or promotion of mobile charge carries and/or reduces or increases the number of the mobile charge carriers. In this manner, the smallest traces of the gas to be detected have an exponential effect on the electrical conductivity of the measuring probe and reduce or increase the conductivity drastically and the effect of a reversible doping occurs on the surface of the measuring probe. This effect occurs even with relatively small concentrations of the agent to be found and is reinforced with increasing concentration of the agent. With an increasing concentration of the agent, this results in a decreasing or increasing differential conductance of the measuring probe with respect to the basic condition. This conductance can, according to the invention and assuming prior calibration, be used for the calculation of the concentration of the agent to be found. To do this an equivalent circuit diagram for the sensor is used, which represents this an electrical dipole. The complex equivalent variables required for the description in the measurement window correlate primarily with the resistances and the thicknesses of the individual layers.